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Organic-based Visible Light Communications

Dr Hoa Le Minh, Prof. Zabih GhassemlooyOptical Communications Research Group(School of CEIS, Northumbria University, UK)http://soe.northumbria.ac.uk/ocr/

TU Graz, Austria – 11th May 2012

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Outline• Visible light communications

• Organic light emitting diode (OLED)

• OLED-based VLC

• Challenges and possibilities discussion

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Visible Light Communications

Ancient time: light was used for medium-range signalling

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Visible Light Communications

Modern time: light is used for high speed communications

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General Light SourcesIncandescent bulb

First industrial light source5% warm light, 95% heatFew thousand hours of life

Fluorescent lampWhite light, cheap25% lightLifetime ~10,000s hours

Solid-state light emitting diode (LED)Compact, cheap, powerful50% lightMore than 50,000 hours lifespan

Organic light emitting diode (OLED)Flexible and bendable panelExtensively used in high-end display products,

HDTV and Smartphone

LED / OLED Devices

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RGB Blue chip + Phosphor OLED

- Well-known technology- Limited use due to

difficulties in RGB balancing

- Phasing out in lighting industry

- Popular for today general lighting industry

- Standardised for illumination and communications

- Emerging technology- Early stage of

development- High potentials

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OLED Current State-of-ArtEfficiency- 100% internal quantum efficiency (Fraunhofer IPMS –

COMEDD, 2012)- Brightness 2.000 cd/m², 5mm thickness (Verbatim Velve,

2012)- 120 lumen (~table lamp) (Philip Lumiblade GL350, 2012)- 80 lumen/watt with 20.000 hours of lifetime (LG, 2012)

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OLED Current State-of-ArtApplications- Dominant in high end Smartphone display products: Super-

AMOLED) (Samsung Galaxy S3 phone, 2012)- 55 inch OLED HDTV (Samsung Electronics, 2012)- 6 inch E-paper on plastic (XGA, 14 gram, 0.7mm thickness),

(LG, 2012)- Solar OLED car (BASF, 2012)- Flexible AMOLED display (Samsung patent, 2012)None of the commercial applications is for communications!

Communications

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200 400 600 800 1000

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Frequency (kHz)

Res

pons

e (d

B)

Measured frequency response of(Philips) Luxeon-star white LED

Measured frequency response of(Philips) Lumiblade white OLED

1. Why OLED modulation bandwidth is too narrow?2. How to improve the OLED bandwidth?

Modulation bandwidth?

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OLED StructureTypical structure:

1. Glass (filled with inert gas to protect other layers)

2. Anode/Hole transport layer (HTL)

3. Organic emitting layers (to control emissive colours)

Including organic compounds4. Electron transport layer (ETL)5. Cathode (typically indium tin oxide (ITO))

Thin film technology:OLED layers ~1-200 nm

Electrical Characterisation

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Rp - electrode contact resistanceRd - diode resistanceC - diode capacitance

For lightingLarge panel better for illumination larger capacitor value

For communicationsLarger capacitor value slow response

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Bandwidth Improvement

- Bandwidth equalisation (Analogue)- Digital filtering- Complex modulation

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Equalisation (First order)

𝐻 (𝜔 )= 1𝑘×

1+ 𝑗 𝜔𝑇

1+ 𝑗 𝜔𝑇𝑘

|𝑯 (𝝎 )|=𝟏𝒌×√ 𝟏+𝝎𝟐𝑻 𝟐

𝟏+𝝎𝟐(𝑻𝒌 )𝟐1

𝑘=

𝑅𝐿

𝑅𝑒𝑞+𝑅𝐿

.

T

H. Le-Minh, D. C. O'Brien, G. Faulkner, L. Zeng, K. Lee, D. Jung and Y. Oh, "100-Mbit/s NRZ Visible Light Communications Using a Post-Equalized White LED", IEEE Photonics Technology Letters, vol. 21, no. 15, pp. 1063-1065, 2009

- Simple implementation- 1st order response (linear)- Cost effective

First order equaliser experimental test-bed

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Equalisation – BW Improvement

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Map of frequency response corresponding to different

equalisers

Philip Lumiblade OLED~£70

H. Le Minh, Z. Ghassemlooy, A. Burton, P. A. Haigh, and S.-K. Liaw, "Bandwidth Improvement for Organic Light Emitting Diodes Based Visible Light Communications", IEEE Communications Letters, 2012 (submit)

H. Le Minh, Z. Ghassemlooy, A. Burton and P. A. Haigh, "Equalization for Organic Light Emitting Diodes in Visible Light Communications" IEEE GLOBECOM, Workshop on Optical Wireless Communications in Houston, USA, 5-9 December, 2011

Equalisation – BER performance

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Measurement/Simulation – 2Mbps NRZ at 400 lux

Experiment – Issue with baseline wandering Need to optimise Tx/Rx circuitry

Improved modulation bandwidth

(experiment)

0 2 4 6 8

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Time (s)

Norm

aliz

ed a

mplit

ude

-0.5 0 0.5 1 1.5-0.2

0

0.2

0.4

0.6

0.8

1

1.2

Time (s)

No

rma

lize

d a

mp

litu

de

Pre-EQ2Post-EQ2Impuls

e respons

e

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Decision Feedback Equalisation

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- DFE: widely used in digital systems transmitting through BW-limited AWGN channels

- Better performance than ZF and MMSE-based filter

𝒛𝒎=∑𝒏=𝟎

𝑵𝟏

𝒄𝒏 𝒚 (𝝁𝑻−𝒏𝝉 )−∑𝒏=𝟏

𝑵𝟐

𝒃𝒏~𝒂𝒎−𝒏

is the sampled incoming signalμT is the μth

sample of the bit period Tzm is the estimated output signal

Osram Orbeos OLED£85

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DFE - Setup

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Parameter ValueData format OOK-NRZPRBS length 2^10 - 1Number of feed-forward taps 18Number of feed-back taps 9Algorithm Least Mean Square (LMS)Algorithm step size 0.03

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DFE - Results

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Unequalised and baseline-wandered RC

equaliser’s BER performance

DFE’s BER performance

Measured BER vs. Bandwidth at different illumination level

(lux)A. Burton, P. A. Haigh, H. Le Minh, Z. Ghassemlooy, S. Rajbhandari and S. K. Liaw, "A Comparative Investigation Study of Modulation and Equalization Techniques for White-Light Emitting Organic Light Emitting Diodes Using in Visible Light Communications", IEEE Communications Magazine, 2012 (submitted)

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Complex Modulation

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Multiple carrier modulation: Orthogonal Frequency Division Multiplexing- Carriers are orthogonal to each others- Each carrier is modulated by QAM, PSK etc.- Equalisation in small band of modulation bandwidth is feasible

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OFDM

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1𝑇 𝑠𝑦𝑚

∫0

𝑇𝑠𝑦𝑚

𝑒 𝑗2 𝜋 𝑓 𝑘𝑡 𝑒−2 𝜋 𝑓 𝑖 𝑡 𝑑𝑡={ 1 ,∧∀𝑘=𝑖0 ,otherwise

Carrier is orthogonal

Data is mapped to QAM signal

A cyclic prefix (CP ) is added to protect from multipath effects  

A. Burton, P. A. Haigh, H. Le Minh, Z. Ghassemlooy, S. Rajbhandari and S. K. Liaw, "A Comparative Investigation Study of Modulation and Equalization Techniques for White-Light Emitting Organic Light Emitting Diodes Using in Visible Light Communications", IEEE Communications Magazine, 2012 (submitted)

OLED OFDM/QAM-based VLC system

P. A. Haigh, et. al., "Exploiting Equalization Techniques for Improving Data Rates in Organic Devices for Visible Light Communications", IEEE Journal of Lightwave Technology, 2012 (submitted)

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OFDM

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Osram Orbeos OLED

1 Mbit/s

3 Mbit/s

5 Mbit/s

@ ~400 lux

64 OFDM carriers, 16-QAM

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OFDM

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Measured BER performance (no FEC) of OLED at ~400 lux

64 OFDM carriers, 16-QAM, raw OLED bandwidth 93 kHz

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Challenges

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• OLED is under development, therefore challenges are widely expected from

- Materials and device structures are being evolved and varied from different manufacturers

- Heavily calibrated for display purpose (unlike LED used for signalling and illumination)

- In the early stage of lighting and decoration utilisation (@2012)

- Expensive (~10/20 times costlier than the same performing LED)

- Lack of wide range of commercially available products

• Communications aspects- Light efficiency is low large illumination panels are

typically fabricated high capacitance thus limiting the device modulation bandwidth (100’s kHz)

- Limited researches in data communications- Not yet being standardised

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Possibilities and Potentials

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• Possibilities and Future Work- Achieving higher data rate, such as 10-15 Mbit/s, so that

OLED can be adopted in standard 10BASE-T Ethernet communications

- Working with the manufacturers to improve the device response time (newer display has faster response and wider dynamic contrast range)

- Device modelling and characterisation to optimise the performance

- Possible to adopt the existing VLC standard (IEEE 802.15/16)- FEC inclusion

• Potentials and Opportunity- OLED is more available in many displays, tablets and phones

new areas of short-range and personal VLC applications and researches

- Toward mobile and flexible VLC- Environmental friendly potentially to be adopted in wide

range of VLC

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Acknowledgement

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OCRG’s OLED / VLC team

Paul A. Haigh

Andrew Burton

Dr. Sujan Rajbhandari

Prof. Erich LeitgebInstitute Hochfrequenztechnik, TU Graz

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Thank you